Related Substances Method Validation - Limit of quantitation

When validating a new drug product Related Substances Method - I have seen methods where LOQ solutions have been prepared and simply injected as is but also have seen LOQ solutions prepared and injected in Mobile Phase and Placebo.

Can it be explained the rationale behind spiking into Mobile Phase or Placebo Please?

In methods received for transfer we do see requirements for checking LOQ in placebo
Could be to check interference of placebo or behavior of finished goods

Dear Michael

There is no ration as such this can be taken as added precaution may be, no regulatory ask to perform this activity as you are already providing the specificity where you are injecting the blank and placebo and proving the specificity. In case of mobile phase this is continuously running through the system so spiking and injecting the impurities in mobile phase does not add any practical value. This may be is to add comfort but definitely not required and not asked for from the regulatory bodies.